1. Field of the Invention
The present invention concerns security documents with safety features in graphical form, preferably using intaglio printing, that can be caused to illuminate as points, strips, and/or surfaces. The targeted wavelengths are in the invisible UV range up to the range visible to the human eye, typically from 360 to 780 nm, as well as within the infrared range.
2. Discussion of the Related Art
In German Patent 43 10 082, electroluminescent foils are presented that are manufactured of inorganic, electroluminescent pigments and thermoplastic synthetics by means of extrusion or co-extrusion. In principle, extrusion or co-extrusion of this type of system onto security paper is conceivable, but graphical design options seem to be limited by process logistics. Also, the overall manufacturing process to produce a security document and the associated authenticity testing arrangement required for it appear to be very expensive.
In German Patent 43 15 244, a process is described that produces an electroluminescent film using sputtering technology. This process would also be fundamentally conceivable for the production of security documents, but a production process of this type would be extremely expensive due to the vacuum chambers necessary for this technology. This process would also be very difficult to integrate into a possible manufacturing process and it produces film layers that would have to be equipped with special added layers to withstand the high mechanical requirements of security documents.
In German Patent 41 26 051, a security document is presented having an embedded surface security element (security fibers). This document is designed with multiple layers and exhibits electroluminescent properties. A disadvantage of this arrangement is that a relatively high surface buildup must be taken into account since the electrodes that are necessary to excite the EL substances lie one on top of the other.
U.S. Pat. No. 4,355,300 shows a testing device to mechanically test the authenticity of a security document. An alternating voltage is applied to an opposing electrode arrangement and produces an electric field between the two electrodes. By placing the security document, with its security features contained on it, between the electrodes, the dielectric constant is modified inside the field gap (the dielectric constant is material-dependent). This changes the capacitive resistance, which can be measured using a measuring instrument and can be evaluated in an evaluation unit. However, this printed document contains no electrically excitable substances that are excited within the field gap and consequently no electromagnetic radiation is measured in the form of emitted photons. The device parameters must therefore be selected according to a measured condenser voltage and not according to a measured electromagnetic radiation.
A primary purpose of the invention, based on German Patent 41 26 051, is to further develop a value/security instrument with EL-active security features that have a considerably thinner layer construction on the surface of the security document.
This objective is met by means of the technical methods described, which provides an essentially planar electrode arrangement located on the value/security instrument. The electrodes lie next to one another in approximately the same plane and form a field gap between them. The field lines of the alternating electromagnetic field produced in the field gap penetrate the EL substances.
In this arrangement, there is a series of different embodiments of the invention. Applying the EL substances using steel plate intaglio printing and other printing processes employ the method of the invention. In particular, dry offset printing, wet offset printing, screen-printing, non-impact printing techniques and novel digital printing processes are among the alternative embodiments.
Instead of layering electrodes one on top of the otherxe2x80x94as the state of the technology is well-known forxe2x80x94these types of electrodes are applied to the value/security instrument and/or testing device in an adjacent surface arrangement, at least partially. An advantage of the invention is thus the elimination a multi-layered arrangement of surface EL systems on top of one another.
In the present state of the technology, the fear is that the known laminate construction, which is subject to extreme sustained stress requirements, may not be sufficiently wear resistant. Another disadvantage is that a security fiber is not an integral component of a value/security instrument and can be removed. This known arrangement requires electrical contacts to be applied to the value/security instrument. In contrast, some of the embodiments of the invention do not need a contact on the value/security instrument.
In contrast to common electroluminescent (referred to as EL) systems that are built between surface electrodes, one variation of this invention does away with this relatively thick construction by building the electric field laterally, or within the surface itself. For an EL plate-condenser type construction according to the invention (in which, according to the invention, the condenser xe2x80x9cplatesxe2x80x9d lie next to one another in essentially the same plane and the field needed for excitation is produced in the field gap between them), a transparent, electrically conducting layer is needed. This layer is attained using ITO pastes (Indium-Tin-Oxide). It can also be attained using pre-layered transparent foils or glasses.
Typically, biaxially-oriented, thermally stabilized polyester foils are used that are layered with evaporated or sputtered, electrically conducting tin oxide, ITO or, generally speaking, transparent, electrically conducting metallized surfaces. These surfaces have surface resistance values in the range of typically 20 ohms/square up to 300 ohms/square and above, and less than this for glass substrates.
High quality EL systems need an even luminous density and maximum light efficiency. Because of their high thermal strength in layering processes, glass substrates offer a higher quality solution, in general, with higher optical transmittance in the visible wavelength range with simultaneously better surface conductivity. However, the important advantage of the ITO paste printing technique, which this invention uses, is its relatively simple application and the nearly limitless graphical design possibilities. This is especially advantageous in more complex systems involving electrical connections.
Since these types of ITO screen printing pastes seldom yield surface resistances of less than 300 to 400 ohm/square, bus bars (edge strips with good electrical conductivity) are used in this invention. This creates even electrical fields and thus an even luminous density. Furthermore, this technique allows the connection of the ITO electrode to be structured with good functionality. Last but not least, the ITO electrode layer thickness can be reduced. This leads to higher transparency through its volume. According to the invention, bus bars are printed by means of printing technology using pastes of silver, carbon, copper, among others, or a combination of these elements. In this way, surface resistances in the range of a few 10 mohms/square can be attained.
According to the invention, various designs are described as follows. In each of these designs, the EL substances are excited using an alternating electromagnetic field.
1. A lateral electrode arrangement on the value/security instrument,
2. An electrode arrangement in a lateral or opposing arrangement located external to the security document, that is, in a signal detector.
3. A lateral electrode arrangement located on a transparent covering substrate in the signal detector.
In a preferred design, the value/security instrument has security elements based on microencapsulated inorganic group II and group VI compounds as found in the periodic table (for example ZnS, CdS). They are doped or activated with metals such as Cu, Mn, Ag, and are suitable for printing using intaglio printing. Also, electroluminescent security elements can be constructed on a base of organic polymers.
The electrodes are designed laterally (that is, lying flat beside one another) using conductive intaglio dyes. An alternating electromagnetic field is set up in the field gap that results between the electrodes. This gap is also flat. The field lines of this field penetrate the printed picture produced by the EL substances, at least partially, and thus cause the electroluminescent security elements to illuminate. These elements can then be subject to visual and machine authenticity testing.
In a preferred embodiment form, electrically conducting intaglio dyes are used that are based on: carbon and/or silver or a mixture of the two, or; silver- and/or gold-coated metallic pigments or mica pigments along with suitable binders based on polyurethanes and/or aliphatic polyesters and appropriate thinners. In particular, the two electrode connections are designed as a non-oxidizing surface.
An aqueous polyurethane layer (preferred) is applied to the unprepared surface of the security documentxe2x80x94preferably a gold certificatexe2x80x94as a dielectric and isolation layer prior to the actual graphical structuring. Then, the EL paste is pressed on. This results in a good, elastic bond with excellent surface durability.
In this process, the preferred graphical form of the luminescent security feature consists of individual points and lines. Moreover, translucent dyes that are appropriately structured (graphically) can be applied above/below/next to the luminescent elements. In this way, different colored light effects can be attained.
A process to manufacture the security document in accordance with this invention involves the following process steps:
Graphical structuring of the substrate (specifically, special security paper with a surface weight of approximately 80 to 200 g/m2) using a graphical printing process, in particular intaglio printing, dry offset printing, wet offset printing, screen printing, non-impact printing and by means of other novel, digital printing processes.
Printing of an adhesive agent, if needed, in the form of an aqueous polyurethane dispersion in order to optimally bind and embed the printed layers that are to follow.
Printing of the lateral electrodes using conducting pastes and, depending on the system, repeating this process more than once to achieve a surface resistance that exhibits sufficient current to suit the chosen geometry of the security elements or that exhibits sufficiently low surface resistance.
Printing of an isolation dye, in particular one with the properties of high elasticity and good bonding to the substrate, the conducting layer and the connected EL dyes, and which has as high a dielectric constant as possible. Aqueous polyurethane dispersion systems are particularly useful, which can be charged with barium titanate (BaTiO3) to raise the dielectric constant.
Printing of the EL paste or the multi-colored illuminating phosphor pastes, with the addition of a range spacer, if needed, that can prevent the microencapsulated light pigments from being damaged at high pressures during the printing process.
If necessary, printing of translucent dyes to create additional graphical and security structure.
If necessary, printing of passivating, electrically conducting dyes onto the electrical connection points. These come in the form of special conducting dyes or pastes based, in particular, on carbon and gold.
Printing of an elastic, transparent, wear-resistant and well adhering protective layer based, in particular, on aqueous polyurethane dispersions.
Hardening of the above printed materials at the end of the printing process.
Optionally, a form of thermal pressing can be done as the last step at temperatures up to 200xc2x0 C. and pressures up to 500 N/cm2 in order to stabilize and improve the quality of the security document.
The preferred design of the invention is characterized according to the above description in that, among other things, lateral electrodes are first arranged using electrically conducting intaglio dyes or pastes and intaglio techniques so that geometries are achieved with extremely high resolution or fineness in the printed picture. These geometries result in high electric field strengths, enabling electroluminescent excitation of typical zinc sulfide phosphor layers.
In this sense, the intaglio print technique proposed by the invention is a very favorable solution due to the extremely high resolution and the multiple xcexcm layer-thickness that can be achieved. However, the structures of the different intaglio dyes or pastes need to be specially adjusted to considerably smaller pigment diameters compared with screen-print dyes.
In this invention, it is important that microencapsulated elements with EL phosphor pastes are used in the intaglio printing process. Capsule diameters of a few xcexcm (for example in the range of 0.2 to 40 xcexcm) are used here.
In a further development of the design, novel substances are used, namely luminescent substances based primarily on silicates, phosphates, tungstates, germanates, borates, among others, that are activated by Mn. However, substances based on Zn2SiO4:Mn (typical substances for fluorescent tubes) are especially preferred.
In addition, certain EL substances can be blocked by UV filter layers, which are in the form of print dyes. For example, using TiO2xe2x80x94filled pigments, any excitation of the EL substances caused by UV light is blocked. This will allow excitation only within the electromagnetic field. This is recommended mainly for the mechanical testing of the security document using the testing device according to the invention. In this device, in a preferred design, visible light is no longer used to perform the test.
In addition, a testing device of a general type to visually and mechanically test the authenticity of value/security documents is to be created. This testing device should allow quick, safe testing of value/security documents and should be simple to operate.
In a first embodiment of the invention, the testing device has two supports parallel to one another. The document to be tested is placed between these supports, at least one of which is transparent. At least one of the supports has an electrode arrangement that is capable of producing an alternating electromagnetic field. This field penetrates the value/security instrument, at least at the points where the EL-active security features are located.
Another embodiment exists in which the testing device has two supports parallel to one another between which the document to be tested is placed. At least one of the supports is transparent. Both supports have electrodes arranged on them that produce an alternating electric field between them similar to a plate condenser. This field penetrates the value/security instrument, at least at the points where the EL-active security features are located.
A third design consists of a testing device with two supports parallel to one another between which the document to be tested is placed. At least one of the supports is transparent. One electrode is located on the value/security instrument and a second electrode is located on one of the supports. These electrodes produce an alternating electromagnetic field between them that penetrates the value/security instrument, at least at the points where the El-active security features are located.